Atlantic-origin water extension into the Pacific Arctic induced an anomalous biogeochemical event

The Arctic Ocean is facing dramatic environmental and ecosystem changes. In this context, an international multiship survey project was undertaken in 2020 to obtain current baseline data. During the survey, unusually low dissolved oxygen and acidified water were found in a high-seas fishable area of the western (Pacific-side) Arctic Ocean. Herein, we show that the Beaufort Gyre shrinks to the east of an ocean ridge and forms a front between the water within the gyre and the water from the eastern (Atlantic-side) Arctic. That phenomenon triggers a frontal northward flow along the ocean ridge. This flow likely transports the low oxygen and acidified water toward the high-seas fishable area; similar biogeochemical properties had previously been observed only on the shelf-slope north of the East Siberian Sea.


Supplementary Discussion 1: Source of the anomalous water on the Chukchi Plateau
Here, we compare vertical sections of hydrographic and biogeochemical parameters across two seas, the Chukchi and East Siberian seas, to identify the source of anomalously low oxygen saturation (dissolved oxygen) and acidified water found on the Chukchi Plateau (CP) in 2020 (Fig. 3).The vertical section from the Chukchi shelf-slope to the southwestern side of the CP (Supplementary Fig. 1a) shows that the anomalously low oxygen saturation water appeared on the seafloor north of 74° N. The low oxygen saturation corresponded with low NO (Supplementary Fig. 1b), where NO is defined as 9[NO3 − ] + [O2] (µmol kg −1 ) and used as a quasi-conservative tracer that is independent of biological processes 1 .Thus, the low oxygen saturation and low NO water are assumed to be delivered from a remote region rather than formed by local microbial oxygen consumption.
Low oxygen saturation and acidified water were found on the East Siberian Sea (ESS) shelf with salinity of about 30-32 and on the shelf-slope where salinity is 34-34.5 (Supplementary Fig. 2a,     b) (ref. 2 ).The low oxygen saturation and acidified water on the shelf-slope could be formed by brine rejection in the ESS shelf 3 and/or through a long-time contact of the water with the shelfslope sediments 4 .The low oxygen saturation and acidified water in the ESS are also characterized by low NO, which is likely caused by the input of river water containing low preformed nitrate, sedimentary denitrification, and decomposition of terrestrial organic matters that are relatively nitrogen-poor compared to marine organic matters 5 .In addition, the ratio NO/PO, where PO is another quasi-conservative tracer defined as 135[PO4 3− ] + [O2] (µmol kg −1 ) 1 , likely has a value less than 0.75 in the ESS, which is the lowest value among the pan-Arctic seas 6 .Likewise, ESS shelf-slope water was characterized by low NO values (200-300 µmol kg −1 ; Supplementary Fig. 2c) and NO/PO < 0.75 (Supplementary Fig. 2d).The anomalously low oxygen saturation and acidified water beside the CP were also low in NO (~230 µmol kg −1 ; Supplementary Fig. 1b) and NO/PO = ~0.66(not shown).Therefore, ESS shelf-slope water with salinity = 34-34.5,NO = 200-300 µmol kg −1 , and NO/PO < 0.75 was probably transported to the CP, on which we found water with similar characteristics (anomalously low oxygen saturation and acidified water with salinity = ~34.5,NO = ~230 µmol kg −1 , and NO/PO = ~0.66).

Supplementary Discussion 2: Interpolation uncertainties
We created gridded datasets using an optimal interpolation method with interpolation uncertainties (see Methods).Uncertainties in dynamic height (Supplementary Fig. 3a-c) were large in the southern branch of the Beaufort Gyre (BG) over the Chukchi shelf-slope, where many eddies were noted to emerge 7,8 .Due to the presence of eddies, the dynamic height varied in space and time, resulting in significant interpolation uncertainties.These uncertainties were relatively small in the 1950s-1980s because the slow BG circulation and its accompanying eddy activity was weak.In 2017-2020, the uncertainty was relatively large in the northward flow along ~170° W, located on the western side of the CP.This northward flow occurred at a front between Pacific Water (PW) and Lower Halocline Water (LHW) caused by the penetration of LHW.The penetration of LHW is shown as a depression of the dynamic height in 2017-2020 compared with that in 2008-2016 west of the CP (Supplementary Fig. 4a).In this area, the depression was larger than the interpolation uncertainty.Hence, we can safely mention that the LHW penetration occurred in 2017-2020, accompanied by the formation of frontal northward flow.
Uncertainties in the vertical salinity sections (Supplementary Fig. 3d-f) were significant in the halocline, where salinity increases sharply with depths at the surface and in an intermediate layer with salinity between 33 and 34.Except for the surface, the uncertainty was largest in the halocline above the CP in 2017-2020 because the front between the PW and LHW was formed and fluctuated around the CP.Increasing salinity, which eventually became larger than the uncertainty, appeared at depths of 100-150 m west of the CP in 2017-2020 (Supplementary Fig. 4b).This salinity increase was caused by shoaling of the halocline, consistent with the LHW penetration to the west of the CP in 2017-2020.
Uncertainties in oxygen saturation (Supplementary Fig. 3g-i ) were prominent at the locations with the lowest oxygen saturation in each period.In 2008-2016 and the 1950s-1980s, the oxygen saturation was lowest along the ESS shelf-slope, increasing sharply toward the north.This significant spatial variation in oxygen saturation resulted in large interpolation uncertainties.On the contrary, in 2017-2020, oxygen saturation was the lowest, and the uncertainty was the largest along ~170° W.During this period, water with anomalously low oxygen saturation was only identified on the CP (near 170° W) in 2020.This temporal variation in oxygen saturation caused an enlargement of the uncertainty at that location.Differences in oxygen saturation between 2017-2020 and 2008-2016 indicate that the oxygen saturation increased along the ESS shelf-slope and decreased along ~170° W (Supplementary Fig. 4c).The decrease in oxygen saturation along ~170° W was smaller than the uncertainty associated with the large temporal variation in oxygen saturation. is abbreviated as BGEP.The 1994 Arctic Ocean Section implemented by the CCGS Louis S. St-Laurent is expressed as AOS 1994.The Chukchi Borderland Project and International Siberian Shelf Study carried out by the United States Coast Guard Cutter Polar Star (USA) in 2002 and Yacob Smirniskyi (Russia) in 2008, respectively, are presented as CBL 2002 and ISSS 2008.